- Email: [email protected]
Investigation of photoneutron production by Siemens artiste linac: A Monte Carlo Study
Author’s Accepted Manuscript Investigation of Photoneutron Production by Siemens Artiste Linac: A Monte Carlo Study Navid Khaledi, Moloud Dabaghi, Dariush Sardari, Farhad Samiei, Foad Goli Ahmadabad, Gholamreza Jahanfarnia, Mohsen Kheradmand Saadi www.elsevier.com/locate/radphyschem
PII: DOI: Reference:
S0969-806X(18)30283-4 https://doi.org/10.1016/j.radphyschem.2018.06.006 RPC7881
To appear in: Radiation Physics and Chemistry Received date: 3 April 2018 Revised date: 18 May 2018 Accepted date: 5 June 2018 Cite this article as: Navid Khaledi, Moloud Dabaghi, Dariush Sardari, Farhad Samiei, Foad Goli Ahmadabad, Gholamreza Jahanfarnia and Mohsen Kheradmand Saadi, Investigation of Photoneutron Production by Siemens Artiste Linac: A Monte Carlo Study, Radiation Physics and Chemistry, https://doi.org/10.1016/j.radphyschem.2018.06.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Investigation of Photoneutron Production by Siemens Artiste Linac: A Monte Carlo Study
Navid Khaledi a,* , Moloud Dabaghib, Dariush Sardari b , Farhad Samieic, Foad Goli Ahmadabadd, Gholamreza Jahanfarniab, Mohsen Kheradmand Saadib
a
b
Department of Radiation Oncology, Novin Medical Radiation Institute, Tehran, Iran
Department of Nuclear Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran c
Radiation Oncology Department, Cancer Institute, Tehran University of Medical Sciences, Tehran, Iran d
School of Medicine, Jiroft University of Medical Sciences, Jiroft, Iran
*Corresponding author: Tel:+98-2188371466, Email:[email protected]
Abstract.
In radiotherapy with electron or photon beams, if the produced photon energy is higher than ~7MeV, neutrons may be produced through photoneutron interaction, exposing not only the patient but the personnel outside the room, by passing through the walls or skyshine. This exposure to photoneutron induced doses can be detrimental to people’s health.
1
In this study, deposited energy and fluences at different points inside and outside the room have been examined by the MCNP code for a Siemens Artiste in a bunker. Different thicknesses of the roof have been investigated for examining the skyshine. In addition, two layouts of room door were compared in term of ambient dose equivalent (ADE) leakage. The deposited energy and fluence distributions were obtained in the head of linac and the bunker.
In the main hall of the room the fast neutrons were prevailing, but within the maze and at the corners, the neutron spectrum has been shifted into thermal and epithermal neutrons (<10 keV). In addition, the peak of the neutron spectrum in front of the head of the linac was about 0.5 to 1.0 MeV. The optimum thickness of the roof was 1 m to reduce skyshine ADE to an acceptable value. The dominant neutrons inside the room were the fast neutrons. In term of ADE, in the outside of the doors, the priority was to lateral door layout.
The findings had a good coincidence with literatures, despite the differences in the room size and linacs models. These characteristics make also possible to calculate the deposited energy distribution and particle spectra for other settings. In addition, the optimum roof thickness calculating can lead to economic and practical effectiveness. Keywords: Photoneutrons, Neutrons, linac, Fluence, Ambient Dose Equivalent, Skyshine
1. INTRODUCTION
The photons produced by the linear accelerator besides the therapeutic aspects, produce unwanted neutrons (photoneutrons)[1-4]. These neutrons are the most important secondary particles in this case, the importance of which is due to high LET of the neutron and its penetration depth
[5]
.
Most of the photoneutron (PN) produced is related to the head components of the device, often composed of heavy and semi-heavy elements such as tungsten, iron, lead, and uranium
[6]
. Some
neutrons are also produced by the concrete of the wall and the roof of the room as well as the patient's own body. The produced neutrons influence on the dose reached to the patient in the room and, in the event of leakage from the room can be very high to the personnel outside the room. Even over time, they also have a destructive effect on the semiconductor elements of electronic equipment. In 2004, Zanini et al.[4] simulated the Monte Carlo (MC) PNs produced by a Varian linac. Their simulation was limited to points around the central axis with a maximum distance of 15 cm. Maximum energy of produced neutrons was 10 MeV. In 2005, Pena et al.[5] compared two types of treatment room design for Siemens PRIMUS linac. The photon energy in their research was 15MV. They found that the fluence of produced neutrons depends on the volume of the room and has a gas nature. They did this for three classes of thermal, semi-thermal and fast neutrons. They found that the dose of the neutron in the IMRT mode is about 2 to 3 times higher than normal. In 2011, Vega et al.
[3]
studied the spectra of PNs around an 18MV accelerator machine.
They obtained the neutron spectrum at different distances from the radiation center. Their obtained spectrum had two peaks, one at around the energy of 1MeV and the other in the thermal region. In 2016, Yücel et al.[7] examined the neutron doses reached to the patient through an 18MV accelerator. They performed dosimetry by foil activation in accelerator isocenter. In their study, neutron fluence was (1.17 ± 0.06) × 107 n / cm2 per Gy. They also did their research for 10×10cm2, 15×15cm2, and 20×20cm2 fields. They found that doses resulted from neutron were not dispensable doses, and therefore, in contrast, they should be taken into account, so that it may even be better to use neutron absorbent materials during treatment.
In 2017, Gracanin et al. [8] performed the doses delivered to the patient's body by MCNP and GEANT4 codes calculations and using a silicon p-i-n diode detector. They found that the diode detector they used was a fast and reliable device for this purpose. The present study deals with the deposited energy, fluence, and energy spectrum of neutrons inside and outside of the bunker by the MCNP MC Code, including the point ambient dose equivalent (ADE) and fluences at the various points, and skyshine calculations for different roof thicknesses as well. In addition, two layouts of room doors were compared together in term of the ADE at the outside of the doors.
2. MATERIALS AND METHODS
First, by using Autocad software[9] and considering the geometry of the linac bunker (Figure 1), and taking into account all thicknesses and geometric shapes, the three-dimensional design of the room and components of the head of the linac (including target, flattening filter, jaws, and primary collimator and shields around the head of the device) was done. The geometry layout was then saved in a ".sat" binary format and imported by the MCNPX software[10] visual environment version 2.6. With the consideration of the atomic number and the weight percent of the components (such as high-density concrete, tungsten, and steel, etc.) in the code, the material section (card) of the code was also completed. Then, by defining the energy spectrum of the electrons colliding to the linac's target and other components, including propagation direction and the distribution function, the source card was also developed. The deposited energy distribution and neutron fluence were investigated in two states of 270 ° and 180 ° Gantry (exposing toward the primary
barrier wall and toward the roof). The doors had a conventional sandwich form design that was made of 0.5, 1.0, 13.0, 1.0, and 0.5 cm lead, steel, polyethylene, steel, and lead sheets, respectively. The MCNP calculations results are per source particle, electron in this study. As a result, the output data (like ADE and fluence) were normalized to the scored dose at the maximum depth (dmax=3 cm) in a water phantom. The normalization factor and the other data used in the calculations are listed in Table 1.
Figure 1- The treatment room plan used in MC simulation: the linac head in 270 ° gantry angle, exposing toward the isocenter (a), primary walls with a thickness of 2.5 m (b), forward (c) and lateral (d) doors layouts, and maze
(e). The location of different point detector tallies and isocenter are illustrated as fn5 and IC, respectively. Each the f115 and f125 tallies are 2 m away from the IC, and located on the central axis of 270 ° gantry angle.
The F5 tally, that is a point detector tally, used for spectrometry of neutrons beams. The F5 tally scores photon or neutron fluence at a certain point that can be converted to ADE by conversion functions feature of MCNPX. In order to calculate the deposited energy distribution, the mesh tally type 1 was used. This mesh tally records the deposited energy or fluence of particles in determined three-dimensional lattices, by the "pedep" or "flux" keywords, respectively. In here, the ADE obtained by ICRP-74[11] conversion functions in MCNPX. Another tally that is being used for dose calculation is F6 tally. The F6 tally scores the energy deposited in a cell. As a result, it is not suitable for dose calculating in an arbitrary section involving various cells. The photo-atomic data from the MCNPLIB04 library
[12]
was used. The cross-section and
fluorescence data were all derived from the ENDF / B-VI.8 data library[13]. The MCNPX variance reduction techniques used in the calculations include: cell importance, biased bremsstrahlung production, biasing photonuclear production, cutoff energy of 7 MeV for electron and photon (threshold energy of most reactions (γ, n)). The bremsstrahlung was biased in graphite, tungsten and copper. Many low-energy photons were produced in the process of bremsstrahlung, but only high-energy (> 7 MeV) photons are of high importance. The variance reduction technique is an efficient method to speed up the calculations by eliminating low impact events, and, focusing on the important ones. However, it may increase the intrinsic uncertainty of calculations, if the scientific concepts are not followed in removing the events. Therefore, in the current study, the energy cutoffs were chosen with regard to the literatures[4,
5]
and photoneutron interaction threshold of linac head materials
[14]
. The minimum
threshold of photoneutron interaction is 7.42 MeV that belongs to tungsten, but our chosen energy cutoff for electron and photon was 7.00 MeV.
To ensure the quality of produced photons in the simulation, the the percentage depth dose (PDD) of 18 MV photon compared with measurement. The peak energy of the incident electrons to the target was changed (tuned) to reach percentage dose deviation of less than 3% between measured and calculated PDD points that was observed for primary electrons peak energy of 17.6 MeV (FWHM=0.8 MeV). For this purpose, PDDs of four Gaussian peak energies (18.0, 17.8, 17.6, and 17.4 MeV) had been compared with the measured one (see Figure 2). The calculated and measured dose, except in the PDD buildup region, have a fairly good consistency, and minor deviations can be observed near the phantom surface. These deviations can be attributed to electron-contamination in photon beam, which is not important in neutron production.
Figure 2- The measured and simulated PDDs for nominal 18 MV photon beam, and four different Gaussian peak energies. The solid line and markers in the main chart correspond to measured and simulated PDD curves, respectively. The markers in the beneath graph represent the percentage dose deviation of each peak energy's PDD related to the measurement.
2.1 Geometrical Structure For simulating the linac head, both full simulation[15] and phase space[16] may be used. However, the phase space is not suitable for neutron contamination calculations; therefore, the full simulation is performed in this study. The geometric structure of the linac and the treatment room were obtained from the file provided by Siemens (Siemens Medical Solutions, Concord, CA). This file contains very detailed information about the accelerator components required for electron and photon simulations. The beam field size was considered 40×40 cm2, to minimize the jaws shielding against the exiting photons and neutrons from the head. Figure 3 illustrates the linac head design for Siemens Artiste. In here, the jaws, primary collimator, and target made of tungsten. Consequently, the target slide, primary collimator slide, and flattening filter are composed of steel. Figure 1 shows the bunker design used in the MC simulation. The primary and secondary walls have 2.5 and 1. 5 m, respectively. The room height was 3 m. In addition, all of the walls and roof were composed of heavy concrete with density of 2.6 g/cm3.
Figure 3- The design of linac head used in the simulation. In here, each adjacent solid grid lines distance is equal to 5 cm. The tungsten target (a), steel flattening filter slide (b), tungsten primary collimator (c), steel flattening filter (d), tungsten leafs and jaws (e), and the lead head shield (f) are the main components of the head in interacting with photon beam.
Photon calculations have been performed for PDD curve and beam profiles in an MP3 phantom (PTW, Freiburg). This phantom is a motorized 50×50×50 cm3 water tank phantom that allows to measure PDD and profile in different directions. The neutron spectrum of components contributions was computed using MCNPX, which required up to 30 million initial photons to produce 10 million PNs. Except for the target (made of tungsten, copper, steel, water and graphite), tungsten components accounted for approximately 87%, and steel components contribute for approximately 2% of total neutron production. The other components are usually at a rate of 0.1% or less[5].
Table 1- The MCNPX parameters used in the calculations.
History numbers
5×108
Electron energy cutoff (MeV)
7 MeV
Photon energy cutoff (MeV)
7 MeV
Photon-atomic data library
MCNPLIB04
Neutron interaction cross section
ENDF/B-VI
Fluence to ADE conversion parameter
IC=40 (ICRP-74)
Mesh tally
Type 1
Point detector tally
F5
Normalization factor (photon dose at dmax , per
4.5×10-16 Gy/e
source electron)
3. RESULTS AND DISCUSSION
In Figure 4a, the normalized deposited energy distribution of neutrons inside the bunker at the 270° Gantry angle shows that the highest deposited energy is produced in the primary walls as well as the head of the linac. Also, in the bunker entrance doors and inside the maze, the lowest deposited energy is produced. Obviously, the energy deposition in the primary wall in the front of the head is more than another primary wall behind the head.
Figure 4- a) Normalized deposited energy distribution inside and outside of the bunker for 270° Gantry. b) Normalized deposited energy distribution inside and outside of the bunker for 180 ° Gantry. c) Fluence distribution inside and outside of the bunker for 270 ° Gantry. d) Fluence distribution inside and outside of the bunker for 180 ° Gantry
As shown in Figure 4b, due to the gantry angle and radiation direction toward the roof, the deposited energy in the walls and inside the maze is decreased. Figure 4c and d show the neutron fluence at 270 ° and 180 ° Gantry angles, respectively. In here, inside the maze, the neutron fluence has fallen due to the secondary wall. In addition, because of the reduction of wall-induced scatters in the 180 ° Gantry angle, the neutron fluence is around 10 times less than 270 ° state.
3.1 Deposited energy and fluence distribution of neutrons inside the head
In Figure 5a and b, it is well demonstrated that the energy deposition is higher in areas that are directly interacting with photon beam and have a high density (such as the target, the jaws, and the head shield). Figure 5c and d, show the fluence inside the head in lateral and upper views, respectively. Despite the similarity with the deposited energy distribution, due to the lower dependence of the neutron fluence on the material and density of components of the head, the distribution is more uniform and smoother. In addition, there is the greatest amount of fluence accumulation in the target.
Figure 5- Lateral (a) and upper (b) views of relative energy deposition distribution, and lateral (c) and upper (d) views of fluence distribution inside the head. Please note that, the energy depositions are normalized to the
maximum value of deposited energy. It helps to see the amount of neutron interactions intensity with the head components visually.
3.2 Ambient Dose Equivalent and fluence in different locations As shown in Figure 1, different detectors (fn5) were placed at different points inside and outside of the bunker, so that f15 tally was used to measure the skyshine for different thicknesses of the roof, and also f75 and f85 tallies were placed to do dosimetry outside of the two types of entrance door designs. The f95, f105, f115, and f125 also provide ADEs at different points within the bunker. Table 2 shows that by increasing the distance from the head of the linac, the ADE and fluence of neutron is decreased. The highest fluence belongs to the two detectors (f115 and f125) that are located on the photon beam central axis. In addition, because of the smaller distance between f125 tally and the head target (1 meter), in comparison with the f115 tally (2 meters), the f125 tally fluence is higher. In addition, the lowest fluence belongs to the f5 detector located behind the primary wall. Furthermore, between the two types of door designs, the lateral type is preferable, because there is less ADE at that point. Figure 6 shows the spectra of neutron fluence in different locations. As it can be seen from Figure 6 a and b the detectors placed in the encounter of the room (f105, f115, and f125) have a high-energy peak between 0.5 and 1.0 MeV. In contrast, Figure 6 c and d show that the f95, f85, f75, and f5 where are beyond the wall, air, or door barriers, have a peak in thermal region (E<0.1 eV). Figure 6 b and d represent the normalized neutron fluence with respect to each spectrum's
maximum value. In the study by Pena et al.
[5]
neutron spectra were obtained at different points (including
corresponding points of f115, f125, f105, and f95). In there, similar to the present work, the neutrons spectra in the encounter of the room was higher than the maze inside and at the corner of
the room. Their spectrum in the main hall had two picks in 1.0 MeV and 0.1 eV, which is similar to Figure 6 a and b. Furthermore, as f95 tally spectrum illustrates a thermal pick in Figure 6 a and b, their spectrum inside the maze had only one peak (in 0.1 eV). In another study[4], the neutron ADE on the central axis was in the mSv Gy-1 range, which is comparable with the f115 and f125 values in Table 2. Moreover, as in other works such as Pena et al.[5] and Zanini et al.[4] , Figure 6a shows that the neutron fluence on the central axis (f115 and f125) is in the 106 neutrons per cm2 range. The existence of neutron fluence peak in thermal area in the outside the door has full agreement with Vega et al.
[17]
. However, in term of intensity, due to the lower energy they used
(10 MV), their peak height was tenth of the current study. The lowest and highest maximum relative error for each energy bin of spectrums inside the room were 0.8% (f115) and 4.3% (f95), respectively. Also in the outside of the room, they were 9.5% (f75) and 19.5% (f5), respectively. These large uncertainties belong to low intensity bins of each spectrum. Consequently, the maximum and minimum uncertainties of ADE were 14.7% (f5) and 0.5% (f115 and f125), respectively. Because of the existence of uncertainty in the fluence-todose functions, the ADEs have higher relative errors than the corresponding net fluences.
Figure 6 – a) neutron fluence and b) normalized neutron fluence with respect to each spectrum's maximum value, at different points of the bunker inside. c) neutron fluence and d) normalized neutron fluence with respect to each spectrum's maximum value, at different points of the bunker outside.
Table 2- The ambient dose equivalent (ADE) and fluence of neutron inside and outside the treatment. Fluence (cm-2 Gy-1)
Tally number
ADE (mSv Gy-1)
f5
1.4×10-3±12.2%
3.2×10-12±14.7%
f75
9.4×101±7.1%
1.9×10-6±8.4%
f85
7.3×10-1±9.9%
1.8×10-7±11.3%
f95
1.1×104±3.0%
7.3×10-3±4.3%
f105
4.0×105±1.8%
1.9×10-1±2.9%
f115
2.4×106±0.4%
2.4±0.5%±0.5%
f125
5.1×106±0.4%
9.8±0.5%
3.3 Skyshine for different roof thicknesses The skyshine ADE calculated by the f15 tally where placed in 3 m away from the primary wall and 1 m from the floor. The thicknesses of the roof vary from zero to 2.5 meters with steps of half-meter change. That is, 6 different roof thicknesses have been compared. In this case, the gantry angle was 180˚. Table 3shows the ADEs of the neutrons skyshine for different thicknesses of the roof.
Table 3- The ambient dose equivalent due to the skyshine outside the treatment room.
Roof thickness (m)
ADE (nSv Gy-1)
0.0
0.5
317±7.5%
0.9±11.6%
1.0 9.6×10±17.7%
5
1.5 2.8×10±21.1%
6
2.0 9.4×10±24.5%
6
2.5 1.9×10±32.8%
7
According to Table 3, and by assuming a dose-rate of 300 monitor unit (MU) per minute, and 100 MU Gy
-1
(3 Gy min-1) and considering the dose limit of 1 mSv y-1 (~ 1.9 nSv min-1) for
members of public [18], the minimum roof thickness should be 1 m. As mentioned previously, reaching small number of particles to the detectors (tallies) lead to having larger uncertainty. Since the f15 tally, which assigned for skyshine calculation, has a long distance from the linac head, therefore, the number of detected neutrons are low. As a result, the uncertainty of skyshine ADE with the 2.5 m roof is very large (32.8%). However, this ADE is smaller than millions of dose limit value for public people, hence its large uncertainty can be neglected.
4. CONCLUSIONS
In the present study, the neutron contamination of an 18 MV Siemens Artiste linac has been investigated using MC method. To normalize the results, the ADE and fluence have been presented per Gy of photon beam at dmax. The neutron fluence inside the maze in 270˚ gantry angle was 10 times higher than 180˚ state. Furthermore, the maximum neutron fluence was 8.5×107 cm-2 Gy-1 that appeared within the head of the linac. The ADE on the photon exposure central axis was in the rage of mSv Gy-1, but in the maze was 7.9 µSv Gy-1. In addition, the ADE behind of the lateral layout door was 10 times less than forward layout, which was 1.9 nSv Gy-1. Consequently, the ADE behind the primary wall was millionth of doors behind and hence it is preferable for assigning an operator room than near the doors. By considering public members, the ADE due to the skyshine was fallen to acceptable dose limit (1.9 nSv min-1) for the roof thicknesses of 1 m and more.
The dominant neutron energy in the main hall was 1 MeV neutrons, and outside the bunker was thermal neutrons (0.01-1 eV). However, the neutron spectrum inside the maze was a mixture of thermal, epithermal, and fast neutrons.
REFERENCES
1. Followill DS, Nüsslin F, Orton CG. IMRT should not be administered at photon energies greater than 10MV. Medical physics. 2007;34:1877-9. 2. Lin J-P, Chu T-C, Lin S-Y, Liu M-T. The measurement of photoneutrons in the vicinity of a Siemens Primus linear accelerator. Applied Radiation and Isotopes. 2001;55:315-21. 3. Vega-Carrillo HR, Baltazar-Raigosa A. Photoneutron spectra around an 18 MV LINAC. Journal of Radioanalytical and Nuclear Chemistry. 2011;287:323-7. 4. Zanini A, Durisi E, Fasolo F, Ongaro C, Visca L, Nastasi U, et al. Monte Carlo simulation of the photoneutron field in linac radiotherapy treatments with different collimation systems. Physics in medicine and biology. 2004;49:571. 5. Pena J, Franco L, Gomez F, Iglesias A, Pardo J, Pombar M. Monte Carlo study of Siemens PRIMUS photoneutron production. Physics in medicine and biology. 2005;50:5921. 6. d'Errico F, Nath R, Tana L, Curzio G, Alberts WG. In‐ phantom dosimetry and spectrometry of photoneutrons from an 18 MV linear accelerator. Medical physics. 1998;25:1717-24. 7. Yücel H, Çobanbaş İ, Kolbaşı A, Yüksel AÖ, Kaya V. Measurement of photo-neutron dose from an 18-MV medical linac using a foil activation method in view of radiation protection of patients. Nuclear Engineering and Technology. 2016;48:525-32. 8. Gracanin V, Guatelli S, Cutajar D, Cornelius I, Tran LT, Bolst D, et al. A convenient verification method of the entrance photo-neutron dose for an 18 MV medical linac using silicon pin diodes. Radiation Measurements. 2017. 9. Earle JH, Olsen D. Engineering Design Graphics: AutoCAD Release 14: Addison-Wesley Longman Publishing Co., Inc.; 1998. 10. Pelowitz DB. MCNPX user’s manual version 2.5. 0. Los Alamos National Laboratory. 2005;76. 11. Thomas R, Brackenbush L, Dietze G. Conversion coefficients for use in radiological protection against external radiation. ICRP Publication.74:3-4. 12. White M. Photoatomic data library MCNPLIB04: a new photoatomic library based on data from ENDF/B-VI release 8. Los Alamos (NM): Los Alamos National Laboratory; 2003. Report No. LA-UR-03-1019. 13. CSEWG-Collaboration. Evaluated nuclear data file ENDF/B-VI. 8. 2001. 14. Chadwick M, Oblozinsky P, Blokhin A, Fukahori T, Han Y, Lee Y, et al. Handbook on photonuclear data for applications: Cross sections and spectra. IAEA TECH-DOC. 2000;1178. 15. Khaledi N, Arbabi A, Sardari D, Rabie Mahdavi S, Aslian H, Dabaghi M, et al. Monte Carlo investigation of the effect of small cutouts on beam profile parameters of 12 and 14 MeV electron beams. Radiat Meas. 2013;51:48-54.
16. Khaledi N, Aghamiri MR, Aslian H, Ameri A. Tabulated square-shaped source model for linear accelerator electron beam simulation. Journal of cancer research and therapeutics. 2017;13:69. 17. Vega-Carrillo HR, Ortíz-Hernandez A, Hernandez-Davila VM, Hernández-Almaraz B, Montalvo TR. H*(10) and neutron spectra around linacs. Journal of radioanalytical and nuclear chemistry. 2010;283:537-40. 18. Protection R. ICRP publication 103. Ann ICRP. 2007;37:2.
Highlights The skyshine, flux, and dose inside and outside the linac bunker were calculated. The dominant neutron energy in the bunker hall is the fast neutrons. Between two design of room doors, the lateral layout was superior, in term of transmitted neutrons. The skyshine shielding saturated after a thickness of 1.5 m roof.
PII: DOI: Reference:
S0969-806X(18)30283-4 https://doi.org/10.1016/j.radphyschem.2018.06.006 RPC7881
To appear in: Radiation Physics and Chemistry Received date: 3 April 2018 Revised date: 18 May 2018 Accepted date: 5 June 2018 Cite this article as: Navid Khaledi, Moloud Dabaghi, Dariush Sardari, Farhad Samiei, Foad Goli Ahmadabad, Gholamreza Jahanfarnia and Mohsen Kheradmand Saadi, Investigation of Photoneutron Production by Siemens Artiste Linac: A Monte Carlo Study, Radiation Physics and Chemistry, https://doi.org/10.1016/j.radphyschem.2018.06.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Investigation of Photoneutron Production by Siemens Artiste Linac: A Monte Carlo Study
Navid Khaledi a,* , Moloud Dabaghib, Dariush Sardari b , Farhad Samieic, Foad Goli Ahmadabadd, Gholamreza Jahanfarniab, Mohsen Kheradmand Saadib
a
b
Department of Radiation Oncology, Novin Medical Radiation Institute, Tehran, Iran
Department of Nuclear Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran c
Radiation Oncology Department, Cancer Institute, Tehran University of Medical Sciences, Tehran, Iran d
School of Medicine, Jiroft University of Medical Sciences, Jiroft, Iran
*Corresponding author: Tel:+98-2188371466, Email:[email protected]
Abstract.
In radiotherapy with electron or photon beams, if the produced photon energy is higher than ~7MeV, neutrons may be produced through photoneutron interaction, exposing not only the patient but the personnel outside the room, by passing through the walls or skyshine. This exposure to photoneutron induced doses can be detrimental to people’s health.
1
In this study, deposited energy and fluences at different points inside and outside the room have been examined by the MCNP code for a Siemens Artiste in a bunker. Different thicknesses of the roof have been investigated for examining the skyshine. In addition, two layouts of room door were compared in term of ambient dose equivalent (ADE) leakage. The deposited energy and fluence distributions were obtained in the head of linac and the bunker.
In the main hall of the room the fast neutrons were prevailing, but within the maze and at the corners, the neutron spectrum has been shifted into thermal and epithermal neutrons (<10 keV). In addition, the peak of the neutron spectrum in front of the head of the linac was about 0.5 to 1.0 MeV. The optimum thickness of the roof was 1 m to reduce skyshine ADE to an acceptable value. The dominant neutrons inside the room were the fast neutrons. In term of ADE, in the outside of the doors, the priority was to lateral door layout.
The findings had a good coincidence with literatures, despite the differences in the room size and linacs models. These characteristics make also possible to calculate the deposited energy distribution and particle spectra for other settings. In addition, the optimum roof thickness calculating can lead to economic and practical effectiveness. Keywords: Photoneutrons, Neutrons, linac, Fluence, Ambient Dose Equivalent, Skyshine
1. INTRODUCTION
The photons produced by the linear accelerator besides the therapeutic aspects, produce unwanted neutrons (photoneutrons)[1-4]. These neutrons are the most important secondary particles in this case, the importance of which is due to high LET of the neutron and its penetration depth
[5]
.
Most of the photoneutron (PN) produced is related to the head components of the device, often composed of heavy and semi-heavy elements such as tungsten, iron, lead, and uranium
[6]
. Some
neutrons are also produced by the concrete of the wall and the roof of the room as well as the patient's own body. The produced neutrons influence on the dose reached to the patient in the room and, in the event of leakage from the room can be very high to the personnel outside the room. Even over time, they also have a destructive effect on the semiconductor elements of electronic equipment. In 2004, Zanini et al.[4] simulated the Monte Carlo (MC) PNs produced by a Varian linac. Their simulation was limited to points around the central axis with a maximum distance of 15 cm. Maximum energy of produced neutrons was 10 MeV. In 2005, Pena et al.[5] compared two types of treatment room design for Siemens PRIMUS linac. The photon energy in their research was 15MV. They found that the fluence of produced neutrons depends on the volume of the room and has a gas nature. They did this for three classes of thermal, semi-thermal and fast neutrons. They found that the dose of the neutron in the IMRT mode is about 2 to 3 times higher than normal. In 2011, Vega et al.
[3]
studied the spectra of PNs around an 18MV accelerator machine.
They obtained the neutron spectrum at different distances from the radiation center. Their obtained spectrum had two peaks, one at around the energy of 1MeV and the other in the thermal region. In 2016, Yücel et al.[7] examined the neutron doses reached to the patient through an 18MV accelerator. They performed dosimetry by foil activation in accelerator isocenter. In their study, neutron fluence was (1.17 ± 0.06) × 107 n / cm2 per Gy. They also did their research for 10×10cm2, 15×15cm2, and 20×20cm2 fields. They found that doses resulted from neutron were not dispensable doses, and therefore, in contrast, they should be taken into account, so that it may even be better to use neutron absorbent materials during treatment.
In 2017, Gracanin et al. [8] performed the doses delivered to the patient's body by MCNP and GEANT4 codes calculations and using a silicon p-i-n diode detector. They found that the diode detector they used was a fast and reliable device for this purpose. The present study deals with the deposited energy, fluence, and energy spectrum of neutrons inside and outside of the bunker by the MCNP MC Code, including the point ambient dose equivalent (ADE) and fluences at the various points, and skyshine calculations for different roof thicknesses as well. In addition, two layouts of room doors were compared together in term of the ADE at the outside of the doors.
2. MATERIALS AND METHODS
First, by using Autocad software[9] and considering the geometry of the linac bunker (Figure 1), and taking into account all thicknesses and geometric shapes, the three-dimensional design of the room and components of the head of the linac (including target, flattening filter, jaws, and primary collimator and shields around the head of the device) was done. The geometry layout was then saved in a ".sat" binary format and imported by the MCNPX software[10] visual environment version 2.6. With the consideration of the atomic number and the weight percent of the components (such as high-density concrete, tungsten, and steel, etc.) in the code, the material section (card) of the code was also completed. Then, by defining the energy spectrum of the electrons colliding to the linac's target and other components, including propagation direction and the distribution function, the source card was also developed. The deposited energy distribution and neutron fluence were investigated in two states of 270 ° and 180 ° Gantry (exposing toward the primary
barrier wall and toward the roof). The doors had a conventional sandwich form design that was made of 0.5, 1.0, 13.0, 1.0, and 0.5 cm lead, steel, polyethylene, steel, and lead sheets, respectively. The MCNP calculations results are per source particle, electron in this study. As a result, the output data (like ADE and fluence) were normalized to the scored dose at the maximum depth (dmax=3 cm) in a water phantom. The normalization factor and the other data used in the calculations are listed in Table 1.
Figure 1- The treatment room plan used in MC simulation: the linac head in 270 ° gantry angle, exposing toward the isocenter (a), primary walls with a thickness of 2.5 m (b), forward (c) and lateral (d) doors layouts, and maze
(e). The location of different point detector tallies and isocenter are illustrated as fn5 and IC, respectively. Each the f115 and f125 tallies are 2 m away from the IC, and located on the central axis of 270 ° gantry angle.
The F5 tally, that is a point detector tally, used for spectrometry of neutrons beams. The F5 tally scores photon or neutron fluence at a certain point that can be converted to ADE by conversion functions feature of MCNPX. In order to calculate the deposited energy distribution, the mesh tally type 1 was used. This mesh tally records the deposited energy or fluence of particles in determined three-dimensional lattices, by the "pedep" or "flux" keywords, respectively. In here, the ADE obtained by ICRP-74[11] conversion functions in MCNPX. Another tally that is being used for dose calculation is F6 tally. The F6 tally scores the energy deposited in a cell. As a result, it is not suitable for dose calculating in an arbitrary section involving various cells. The photo-atomic data from the MCNPLIB04 library
[12]
was used. The cross-section and
fluorescence data were all derived from the ENDF / B-VI.8 data library[13]. The MCNPX variance reduction techniques used in the calculations include: cell importance, biased bremsstrahlung production, biasing photonuclear production, cutoff energy of 7 MeV for electron and photon (threshold energy of most reactions (γ, n)). The bremsstrahlung was biased in graphite, tungsten and copper. Many low-energy photons were produced in the process of bremsstrahlung, but only high-energy (> 7 MeV) photons are of high importance. The variance reduction technique is an efficient method to speed up the calculations by eliminating low impact events, and, focusing on the important ones. However, it may increase the intrinsic uncertainty of calculations, if the scientific concepts are not followed in removing the events. Therefore, in the current study, the energy cutoffs were chosen with regard to the literatures[4,
5]
and photoneutron interaction threshold of linac head materials
[14]
. The minimum
threshold of photoneutron interaction is 7.42 MeV that belongs to tungsten, but our chosen energy cutoff for electron and photon was 7.00 MeV.
To ensure the quality of produced photons in the simulation, the the percentage depth dose (PDD) of 18 MV photon compared with measurement. The peak energy of the incident electrons to the target was changed (tuned) to reach percentage dose deviation of less than 3% between measured and calculated PDD points that was observed for primary electrons peak energy of 17.6 MeV (FWHM=0.8 MeV). For this purpose, PDDs of four Gaussian peak energies (18.0, 17.8, 17.6, and 17.4 MeV) had been compared with the measured one (see Figure 2). The calculated and measured dose, except in the PDD buildup region, have a fairly good consistency, and minor deviations can be observed near the phantom surface. These deviations can be attributed to electron-contamination in photon beam, which is not important in neutron production.
Figure 2- The measured and simulated PDDs for nominal 18 MV photon beam, and four different Gaussian peak energies. The solid line and markers in the main chart correspond to measured and simulated PDD curves, respectively. The markers in the beneath graph represent the percentage dose deviation of each peak energy's PDD related to the measurement.
2.1 Geometrical Structure For simulating the linac head, both full simulation[15] and phase space[16] may be used. However, the phase space is not suitable for neutron contamination calculations; therefore, the full simulation is performed in this study. The geometric structure of the linac and the treatment room were obtained from the file provided by Siemens (Siemens Medical Solutions, Concord, CA). This file contains very detailed information about the accelerator components required for electron and photon simulations. The beam field size was considered 40×40 cm2, to minimize the jaws shielding against the exiting photons and neutrons from the head. Figure 3 illustrates the linac head design for Siemens Artiste. In here, the jaws, primary collimator, and target made of tungsten. Consequently, the target slide, primary collimator slide, and flattening filter are composed of steel. Figure 1 shows the bunker design used in the MC simulation. The primary and secondary walls have 2.5 and 1. 5 m, respectively. The room height was 3 m. In addition, all of the walls and roof were composed of heavy concrete with density of 2.6 g/cm3.
Figure 3- The design of linac head used in the simulation. In here, each adjacent solid grid lines distance is equal to 5 cm. The tungsten target (a), steel flattening filter slide (b), tungsten primary collimator (c), steel flattening filter (d), tungsten leafs and jaws (e), and the lead head shield (f) are the main components of the head in interacting with photon beam.
Photon calculations have been performed for PDD curve and beam profiles in an MP3 phantom (PTW, Freiburg). This phantom is a motorized 50×50×50 cm3 water tank phantom that allows to measure PDD and profile in different directions. The neutron spectrum of components contributions was computed using MCNPX, which required up to 30 million initial photons to produce 10 million PNs. Except for the target (made of tungsten, copper, steel, water and graphite), tungsten components accounted for approximately 87%, and steel components contribute for approximately 2% of total neutron production. The other components are usually at a rate of 0.1% or less[5].
Table 1- The MCNPX parameters used in the calculations.
History numbers
5×108
Electron energy cutoff (MeV)
7 MeV
Photon energy cutoff (MeV)
7 MeV
Photon-atomic data library
MCNPLIB04
Neutron interaction cross section
ENDF/B-VI
Fluence to ADE conversion parameter
IC=40 (ICRP-74)
Mesh tally
Type 1
Point detector tally
F5
Normalization factor (photon dose at dmax , per
4.5×10-16 Gy/e
source electron)
3. RESULTS AND DISCUSSION
In Figure 4a, the normalized deposited energy distribution of neutrons inside the bunker at the 270° Gantry angle shows that the highest deposited energy is produced in the primary walls as well as the head of the linac. Also, in the bunker entrance doors and inside the maze, the lowest deposited energy is produced. Obviously, the energy deposition in the primary wall in the front of the head is more than another primary wall behind the head.
Figure 4- a) Normalized deposited energy distribution inside and outside of the bunker for 270° Gantry. b) Normalized deposited energy distribution inside and outside of the bunker for 180 ° Gantry. c) Fluence distribution inside and outside of the bunker for 270 ° Gantry. d) Fluence distribution inside and outside of the bunker for 180 ° Gantry
As shown in Figure 4b, due to the gantry angle and radiation direction toward the roof, the deposited energy in the walls and inside the maze is decreased. Figure 4c and d show the neutron fluence at 270 ° and 180 ° Gantry angles, respectively. In here, inside the maze, the neutron fluence has fallen due to the secondary wall. In addition, because of the reduction of wall-induced scatters in the 180 ° Gantry angle, the neutron fluence is around 10 times less than 270 ° state.
3.1 Deposited energy and fluence distribution of neutrons inside the head
In Figure 5a and b, it is well demonstrated that the energy deposition is higher in areas that are directly interacting with photon beam and have a high density (such as the target, the jaws, and the head shield). Figure 5c and d, show the fluence inside the head in lateral and upper views, respectively. Despite the similarity with the deposited energy distribution, due to the lower dependence of the neutron fluence on the material and density of components of the head, the distribution is more uniform and smoother. In addition, there is the greatest amount of fluence accumulation in the target.
Figure 5- Lateral (a) and upper (b) views of relative energy deposition distribution, and lateral (c) and upper (d) views of fluence distribution inside the head. Please note that, the energy depositions are normalized to the
maximum value of deposited energy. It helps to see the amount of neutron interactions intensity with the head components visually.
3.2 Ambient Dose Equivalent and fluence in different locations As shown in Figure 1, different detectors (fn5) were placed at different points inside and outside of the bunker, so that f15 tally was used to measure the skyshine for different thicknesses of the roof, and also f75 and f85 tallies were placed to do dosimetry outside of the two types of entrance door designs. The f95, f105, f115, and f125 also provide ADEs at different points within the bunker. Table 2 shows that by increasing the distance from the head of the linac, the ADE and fluence of neutron is decreased. The highest fluence belongs to the two detectors (f115 and f125) that are located on the photon beam central axis. In addition, because of the smaller distance between f125 tally and the head target (1 meter), in comparison with the f115 tally (2 meters), the f125 tally fluence is higher. In addition, the lowest fluence belongs to the f5 detector located behind the primary wall. Furthermore, between the two types of door designs, the lateral type is preferable, because there is less ADE at that point. Figure 6 shows the spectra of neutron fluence in different locations. As it can be seen from Figure 6 a and b the detectors placed in the encounter of the room (f105, f115, and f125) have a high-energy peak between 0.5 and 1.0 MeV. In contrast, Figure 6 c and d show that the f95, f85, f75, and f5 where are beyond the wall, air, or door barriers, have a peak in thermal region (E<0.1 eV). Figure 6 b and d represent the normalized neutron fluence with respect to each spectrum's
maximum value. In the study by Pena et al.
[5]
neutron spectra were obtained at different points (including
corresponding points of f115, f125, f105, and f95). In there, similar to the present work, the neutrons spectra in the encounter of the room was higher than the maze inside and at the corner of
the room. Their spectrum in the main hall had two picks in 1.0 MeV and 0.1 eV, which is similar to Figure 6 a and b. Furthermore, as f95 tally spectrum illustrates a thermal pick in Figure 6 a and b, their spectrum inside the maze had only one peak (in 0.1 eV). In another study[4], the neutron ADE on the central axis was in the mSv Gy-1 range, which is comparable with the f115 and f125 values in Table 2. Moreover, as in other works such as Pena et al.[5] and Zanini et al.[4] , Figure 6a shows that the neutron fluence on the central axis (f115 and f125) is in the 106 neutrons per cm2 range. The existence of neutron fluence peak in thermal area in the outside the door has full agreement with Vega et al.
[17]
. However, in term of intensity, due to the lower energy they used
(10 MV), their peak height was tenth of the current study. The lowest and highest maximum relative error for each energy bin of spectrums inside the room were 0.8% (f115) and 4.3% (f95), respectively. Also in the outside of the room, they were 9.5% (f75) and 19.5% (f5), respectively. These large uncertainties belong to low intensity bins of each spectrum. Consequently, the maximum and minimum uncertainties of ADE were 14.7% (f5) and 0.5% (f115 and f125), respectively. Because of the existence of uncertainty in the fluence-todose functions, the ADEs have higher relative errors than the corresponding net fluences.
Figure 6 – a) neutron fluence and b) normalized neutron fluence with respect to each spectrum's maximum value, at different points of the bunker inside. c) neutron fluence and d) normalized neutron fluence with respect to each spectrum's maximum value, at different points of the bunker outside.
Table 2- The ambient dose equivalent (ADE) and fluence of neutron inside and outside the treatment. Fluence (cm-2 Gy-1)
Tally number
ADE (mSv Gy-1)
f5
1.4×10-3±12.2%
3.2×10-12±14.7%
f75
9.4×101±7.1%
1.9×10-6±8.4%
f85
7.3×10-1±9.9%
1.8×10-7±11.3%
f95
1.1×104±3.0%
7.3×10-3±4.3%
f105
4.0×105±1.8%
1.9×10-1±2.9%
f115
2.4×106±0.4%
2.4±0.5%±0.5%
f125
5.1×106±0.4%
9.8±0.5%
3.3 Skyshine for different roof thicknesses The skyshine ADE calculated by the f15 tally where placed in 3 m away from the primary wall and 1 m from the floor. The thicknesses of the roof vary from zero to 2.5 meters with steps of half-meter change. That is, 6 different roof thicknesses have been compared. In this case, the gantry angle was 180˚. Table 3shows the ADEs of the neutrons skyshine for different thicknesses of the roof.
Table 3- The ambient dose equivalent due to the skyshine outside the treatment room.
Roof thickness (m)
ADE (nSv Gy-1)
0.0
0.5
317±7.5%
0.9±11.6%
1.0 9.6×10±17.7%
5
1.5 2.8×10±21.1%
6
2.0 9.4×10±24.5%
6
2.5 1.9×10±32.8%
7
According to Table 3, and by assuming a dose-rate of 300 monitor unit (MU) per minute, and 100 MU Gy
-1
(3 Gy min-1) and considering the dose limit of 1 mSv y-1 (~ 1.9 nSv min-1) for
members of public [18], the minimum roof thickness should be 1 m. As mentioned previously, reaching small number of particles to the detectors (tallies) lead to having larger uncertainty. Since the f15 tally, which assigned for skyshine calculation, has a long distance from the linac head, therefore, the number of detected neutrons are low. As a result, the uncertainty of skyshine ADE with the 2.5 m roof is very large (32.8%). However, this ADE is smaller than millions of dose limit value for public people, hence its large uncertainty can be neglected.
4. CONCLUSIONS
In the present study, the neutron contamination of an 18 MV Siemens Artiste linac has been investigated using MC method. To normalize the results, the ADE and fluence have been presented per Gy of photon beam at dmax. The neutron fluence inside the maze in 270˚ gantry angle was 10 times higher than 180˚ state. Furthermore, the maximum neutron fluence was 8.5×107 cm-2 Gy-1 that appeared within the head of the linac. The ADE on the photon exposure central axis was in the rage of mSv Gy-1, but in the maze was 7.9 µSv Gy-1. In addition, the ADE behind of the lateral layout door was 10 times less than forward layout, which was 1.9 nSv Gy-1. Consequently, the ADE behind the primary wall was millionth of doors behind and hence it is preferable for assigning an operator room than near the doors. By considering public members, the ADE due to the skyshine was fallen to acceptable dose limit (1.9 nSv min-1) for the roof thicknesses of 1 m and more.
The dominant neutron energy in the main hall was 1 MeV neutrons, and outside the bunker was thermal neutrons (0.01-1 eV). However, the neutron spectrum inside the maze was a mixture of thermal, epithermal, and fast neutrons.
REFERENCES
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Highlights The skyshine, flux, and dose inside and outside the linac bunker were calculated. The dominant neutron energy in the bunker hall is the fast neutrons. Between two design of room doors, the lateral layout was superior, in term of transmitted neutrons. The skyshine shielding saturated after a thickness of 1.5 m roof.